1. Technical Field
The present invention relates to methods for measurement, analysis and assessment of ground structure.
2. Background Art
To determine the structure and characteristics of the surface layer of the ground, it has heretofore been necessary to drill a plurality of holes in the ground and insert or indwell instruments and ancillary devices into the holes and also to employ a source of oscillation for generating waves to be measured. It should, therefore, be obviously of great benefit if it were ever possible to know the structure and characteristics of the surface layer of the ground without any of such efforts and contrivances. To this end, it might be contemplated to measure and analyze the oscillatory waves or microseisms observed in the surface layer of the ground. However, up to the present time, there is not known a measuring technology or a system of instrumental analysis, by means of which short-period microseisms could be measured with sufficient accuracy to estimate the tectonic and other characteristics of the surface layer of the ground inclusive of the velocity of shear (S) waves.
Furthermore, the prior art system designed to simultaneously measure short-period microseisms at a plurality of points to thereby delineate the ground structure has the following disadvantages.
A. Since the spacings of oscillation sensors must be large, that is to say the scale of measurement must be large, a large free area is required and it is time-consuming to connect the cords from the respective oscillation sensors for central analysis. Moreover, because the large number of oscillation sensors are required, the analytical procedure is scaled up to the extent that the method cannot be utilized advantageously in the field.
B. Since the network of oscillation sensors is not tailored to improvement in measuring accuracy or resolution of high-order modes, the reliability of the measured dispersion curve is low.
C. Short-period microseisms include not only substantive waves such as dilatational (P-) waves and S-waves but surface waves such as Rayleigh and Love waves. Since the confirmation or judgement of whether the detected waves are surface waves or not is not performed at measurement or analysis, it is impossible to judge whether the dispersion curve obtained is relevant to the surface waves and, hence, to accurately determine the ground structure.
D. Since short-period waves include higher-modes surface waves, it is impossible to compute an accurate surface wave dispersion curve and, hence, to ascertain the surface layer structure of the ground unless some means for removing or separating these waves are available.
E. Since the dispersion curve cannot be computed real-time from field values, there is no knowing whether measurements were made with acceptable accuracy.
It is an object of the present invention to provide a method for measurement, analysis and assessment of ground structure whereby the laminar structure and characteristics of the ground can be determined quickly, expediently and accurately through simultaneous ground-level observation and analysis of short-period microseisms even in a confined available land.
It is another object of the present invention to provide a method for measurement, analysis and assessment of ground structure which does not call for drilling of holes or the use of an oscillation source.
It is a further object of the invention to provide a method for measurement, analysis and assessment of ground structure in which the accuracy of measurement and analysis has been improved by using a network of substantially equi-spaced oscillation sensors which facilitates field sensor positioning as well.
It is another object of the present invention to provide a method for measurement, analysis and assessment of ground structure by which the laminar structure and characteristics of the ground can be ascertained with accuracy through selective extraction of surface waves.
It is still another object of the present invention to provide a method for measurement, analysis and assessment of ground structure by which the accuracy of assessment of the ground structure can be improved by a collective analysis of data obtained by repeated measurement using dissimilar sensor-to-sensor distances.
It is still another object of the present invention to provide a method for measurement, analysis and assessment of ground structure by which the laminar structure and characteristics of the ground can be evaluated conveniently and accurately on a real-time basis.
The present invention is predicated on the facts that when surface waves such as Rayleigh waves are propagated through a horizontal multi-layer ground structure, phase velocity (the velocity of propagation) varies with different wavelengths (dispersions), that the apparent phase velocity can be computed by averaging the correlation coefficients, of the waveforms measured by equi-spaced oscillation sensors in different directions, that this apparent phase velocity varies with different sensor-to-sensor differences due to the influence of higher-order modes, and that when the horizontal and vertical components of the motion of a particle on the ground surface due to propagation of oscillations are detected and synthesized, the locus of rotation or orbit of a particle on the ground surface can be constructed, and that such surface waves are present in short-period microseisms as well. As such, the present invention is directed to a method for measurement, analysis and assessment of surface wave dispersion curves which serve as bases for evaluation of the tetonic and elastic characteristics of the ground by repeated simultaneous measurement of short-period microseisms at a plurality of points in a simple small-scale measuring system wherein 3 or more oscillation sensors are disposed at substantially equal sensor-to-sensor distances within a comparatively small area and inverse analysis of the surface wave dispersion curves at the points of measurement to compute the S- wave velocity of the ground and evaluate the laminar structure and features of the ground.
In the first aspect, the present invention provides a method for estimating the laminar structure and characteristics of a ground which comprises disposing an oscillation sensor adapted to detect the vertical component or the vertical and horizontal components of oscillations at each of at least 3 points within a comparatively narrow area and thereby measuring microseisms at such plural points simultaneously, if necessary changing the points of measurement and repeating the measurement, and analyzing the vertical oscillation data or vertical and horizontal oscillation data.
In the second aspect, the present invention is directed to a method for estimating the laminar structure and characteristics of a ground according to said first aspect wherein said microseisms to be measured are short-period microseisms observed at ground level.
In the third aspect, the present invention is directed to a method according to said first aspect wherein two oscillation sensors are disposed on each of 2 or more straight lines passing through the ground to be measured and dividing the azimuth of 360 degrees into an even-number of sections or straight lines paralleled thereto in such a manner that all sensor-to-sensor-distances are equal on each of said straight lines.
In the fourth aspect, the present invention provides a method according to said first aspect wherein said oscillation sensors are disposed at 3 or more points with equal spacings on the circumference of a circle and at one point in the center of the circle, or at a total of at least 4 points.
In the fifth aspect, the present invention provides a method according to said first aspect wherein said oscillation sensors are disposed at apices of a regular triangle and the center of gravity thereof or a total of 4 points.
In the sixth aspect, the present invention is directed to a method for estimating the laminar structure and characteristics of a ground which comprises analyzing vertical oscillation data or vertical and horizontal oscillation data as measured simultaneously with oscillation sensors, judging whether the oscillation waves detected by said oscillation sensors are surface waves or not, and extracting and analyzing the surface waves.
In the seventh aspect, the present invention is directed to a method for evaluating the laminar structure and characteristics of a ground which comprises analyzing the correlation coefficients and phase differences of oscillation data measured by equi-spaced oscillation sensors at respective frequencies, performing the analysis for each of varying sensor-to-sensor distances, analyzing and computing the fundamental mode of surface waves based on results of the respective analyses and, if necessary, analyzing and completing the higher-mode wavelengths.
In the eighth aspect, the present invention relates to a method for estimating the laminar structure and characteristics of a ground which comprises performing the measurement and analysis of microseisms repeatedly until it can be concluded that a relation between phase velocity and wavelength such as to give a sufficiently accurate evaluation of ground structure can be achieved.
Thus, in accordance with the present invention, oscillation sensors for detecting the vertical component or the vertical and horizontal components of a vibration are disposed at 3 or more points within a comparatively small area at ground level and the microseisms are measured simultaneously at said points, if necessary changing the locations of said points of measurement and the resulting vertical oscillation data or vertical and horizontal oscillation data are analyzed. By measuring microseisms with a small network of oscillation sensors, the accuracy of assessment of the surface structure of the ground can be improved and a system enabling a rapid field measurement, analysis and assessment can be constructed. Therefore, the laminar structure and characteristics of the ground can be estimated expediently, positively and accurately.
Furthermore, the present invention is concerned with short-period microseisms which can be easily measured at ground level and as such does not require drilling and placement of instruments in the drilled holes or the use of an oscillation source so that the measurement and analysis of ground structure can be easily performed in any location.
Moreover, since the network of oscillation sensors for measurement can be set up in a comparatively confined area, the measurement is facilitated and can be performed even in a small plot of land.
In addition, when the vertical and horizontal oscillation data are analyzed and the locus of rotation of a particle on the ground surface due to propagation of oscillations is constructed and the analysis and assessment are carried out on confirming that the measured waves are surface waves, the P and S waves can be differentiated and the surface waves only be positively measured to provide accurate information on the ground structure.
Moreover, when oscillation sensors are disposed in a network wherein sensor-to-sensor distances are substantially equal and the measurement of waves is repeated varying the sensor-to-sensor distances, the high-mode surface waves can be isolated from the fundamental mode, with the result that a highly accurate relation of dispersion wave to wavelength can be obtained. It is because the analytical system is constructed using a microcomputer that the assessment can be obtained on the spot.
Moreover, when it is so arranged that the measurement of microseisms, amplification of detected signals and the analysis and assessment of data are automatically performed as a sequence, the relationship between dispersion curve and wavelength can be more accurately determined in the field.
In accordance with the present invention, a plurality of oscillation sensors for detecting the vertical component or vertical and horizontal components of oscillations are arranged at 3 or more points in a comparatively small area, the measurements of microseisms at the respective points are simultaneously performed, if necessary varying the locations of said points and repeating the measurements, and the measured vertical oscillation data or vertical and horizontal oscillation data are analyzed to ascertain the laminar structure and characteristics of the ground. Therefore, the laminar structure and other characteristics of the ground can be determined not only expediently, positively and accurately but even when the area available for measurement is confined.
In the second aspect of the present invention wherein said microseisms to be measured are short-period microseisms observed on the surface of the ground, the laminar structure and other characteristics of the ground can be determined without calling for drilling of holes or the use of an oscillation source in addition to realization of the effects achieved by the first aspect of the invention.
According to the third aspect of the invention, two oscillation sensors are arranged on two or more straight lines passing through the ground to be measured and dividing the azimuth of 360 degrees into an even-number sections or straight lines parallel thereto in such a manner that the distance between the oscillation sensors is constant on each of said straight lines, with the result that, in addition to the effects of the first aspect of the invention, the field installation of the oscillation sensors is facilitated and, moreover, because the sensor-to-sensor distance is constant in different directions, the accuracy of measurement and analysis is improved.
Comparable effects are obtained in the fourth and fifth inventions.
In accordance with the sixth aspect of the present invention, the vertical oscillation data or vertical and horizontal oscillation data measured simultaneously by said oscillation sensors are analyzed to see whether the oscillation waves detected by the oscillation sensors are surface waves or not and the laminar structure and other characteristics of the ground are still more accurately estimated from the accurate dispersion curves constructed by extraction and selective analysis of the surface waves.
In accordance with the seventh aspect of the present invention, the correlation coefficients and phase differences of the oscillation data measured by oscillation sensors disposed at equal spacings for the respective wavelength are analyzed, varying the sensor-to-sensor difference and repeating the measurements and, based on results of the respective analyses, the fundamental mode of the surface waves is analyzed and computed. If necessary the wavelength of the higher mode is analyzed and computed to assess the laminar structure and characteristics of the ground. Therefore, in addition to the effects of the first to the sixth aspect of the invention, the higher mode surface waves which could not be easily isolated in the past can be neatly isolated for insuring a further improvement in the accuracy of assessment of ground structure.
In accordance with the eighth aspect of the invention, the measurement of microseisms and the analysis of measured microseismic data are continued until a relationship between phase velocity and wave-length such that the ground structure can be determined with sufficient accuracy is obtained so that the laminar structure and characteristics of the ground can be determined expediently, rapidly and accurately on a real-time basis.